Collision-driven slab breakoff magmatism in northwestern Anatolia, Turkey

Postcollisional Eocene magmatism in northwestern Anatolia produced two E-W-trending linear belts of plutons along and north of the Izmir-Ankara-Erzincan suture zone (IAESZ), whose geochemical features and age relations support a slab breakoff model for their petrogenetic oinevolution. The suture zone granitoids (SZGs) in the southern belt have ages around 54-48 Ma, are intrusive into blueschist rocks of the IAESZ, and are composed of diorite, quartz diorite, granodiorite, and syenite. The Marmara granitoids (MGs) in the northern belt are slightly younger (48-35 Ma), intrusive into the Paleozoic-Mesozoic crystalline basement rocks of the Sakarya continent, and composed of monzogranite, granite, and granodiorite. Both SZGs and MGs have moderately to highly evolved medium-to high-K calc-alkaline compositions and are predominantly metaluminous I-type granitoids. Nd-Sr isotope systematics (Sr-87/Sr-86((t)) = 0.70624-0.70704; Nd-143/Nd-144((t)) = 0.512430-0.512439) and trace element compositions of the SZGs and MGs suggest a metasomatized lithospheric mantle source modified by the Late Cretaceous subduction event for their parental melts. Partial melting of this mantle lithosphere was facilitated by asthenospheric upwelling and associated thermal perturbation in response to a slab breakoff experienced by the Tethyan oceanic lithosphere subducted beneath the Sakarya continent. Mantle-derived melts were modified by crustal contamination, assimilation, and fractional crystallization processes as they migrated through the overlying crust. Stronger depletion of the MG rocks in Eu, Ba, Sr, and P and their higher contents of Pb, K, Ni, and SiO2 in comparison to the SZG rocks suggest greater amounts of crustal contamination during the evolution of their magmas rising through the Sakarya continental crust. The geochemistry, petrogenesis, and geochronology of the subparallel SZG and MG belts along and north of the IAESZ provide a case study of the evolution of slab breakoff magmatism in an Alpine-style collision zone.